Complement consists of a group of over 20 serum proteins which constitutes a major part of the immune defense mechanism against certain parasitic diseases such as malaria and trypanosomiasi, some bacterial infections (Vibrio cholerae and Salmonella typhosa), fungi and viruses. The end result of complement action in many circumstances is irreversible membrane damage to the cell under complement attack, leading ultimately to cell death. Although the protein components making up the membrane lesion are known, their mode of action against a target membrane is unclear. My previous findings using a photoreactive lipid probe that was anchored within the hydrophobic region of a model target membrane indicate that all five of the terminal complement proteins (C5b-C9) penetrate at least the outer leaflet of the membrane during complement attack. In addition, the buildup of the final complement lesion is accompanied by a drastic rearrangement of proteins with respect to the lipid bilayer of the membrane. The specific goals of this project are: 1) to elucidate the final structure of the complement lesion and the molecular rearrangements leading up to it, 2) to define the physicochemical properties of the target membrane that alter susceptibility to C attack and 3) to develop a simple structural model to study the mechanism of lesion formation by C8 and C9. The topographical arrangement of the proteins making up the terminal complex will be investigated using both hydrophobic photoreactive lipid probes restricted to either the inner or outer monolayer of the target membrane, as well as surface-restricted probes on inside-out membranes to identify complement proteins that penetrate all the way through the membrane. With these techniques it will be possible to follow movement of each of the membrane attack proteins within or on the target membranes as the chemical and physical properties of the membrane are systematically changed and to correlate the insertional capability of the terminal proteins with a target's susceptibility to attack by complement. The long-range goals of this proposed research are to provide a clearer understanding of the mechanism of complement lysis and to suggest possible regimens for the treatment of complement-related disorders.